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customizing-unicode-functions.html
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customizing-unicode-functions.html
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>Customizing Unicode functions: HarfBuzz Manual</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="customizing-unicode-functions"></a>Customizing Unicode functions</h2></div></div></div>
<p>
HarfBuzz requires some simple functions for accessing
information from the Unicode Character Database (such as the
<code class="literal">General_Category</code> (gc) and
<code class="literal">Script</code> (sc) properties) that is useful
for shaping, as well as some useful operations like composing and
decomposing code points.
</p>
<p>
HarfBuzz includes its own internal, lightweight set of Unicode
functions. At build time, it is also possible to compile support
for some other options, such as the Unicode functions provided
by GLib or the International Components for Unicode (ICU)
library. Generally, this option is only of interest for client
programs that have specific integration requirements or that do
a significant amount of customization.
</p>
<p>
If your program has access to other Unicode functions, however,
such as through a system library or application framework, you
might prefer to use those instead of the built-in
options. HarfBuzz supports this by implementing its Unicode
functions as a set of virtual methods that you can replace —
without otherwise affecting HarfBuzz's functionality.
</p>
<p>
The Unicode functions are specified in a structure called
<code class="literal">unicode_funcs</code> which is attached to each
buffer. But even though <code class="literal">unicode_funcs</code> is
associated with a <span class="type">hb_buffer_t</span>, the functions
themselves are called by other HarfBuzz APIs that access
buffers, so it would be unwise for you to hook different
functions into different buffers.
</p>
<p>
In addition, you can mark your <code class="literal">unicode_funcs</code>
as immutable by calling
<code class="function">hb_unicode_funcs_make_immutable (ufuncs)</code>.
This is especially useful if your code is a
library or framework that will have its own client programs. By
marking your Unicode function choices as immutable, you prevent
your own client programs from changing the
<code class="literal">unicode_funcs</code> configuration and introducing
inconsistencies and errors downstream.
</p>
<p>
You can retrieve the Unicode-functions configuration for
your buffer by calling <code class="function">hb_buffer_get_unicode_funcs()</code>:
</p>
<pre class="programlisting">
hb_unicode_funcs_t *ufunctions;
ufunctions = hb_buffer_get_unicode_funcs(buf);
</pre>
<p>
The current version of <code class="literal">unicode_funcs</code> uses six functions:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
<code class="function">hb_unicode_combining_class_func_t</code>:
returns the Canonical Combining Class of a code point.
</p></li>
<li class="listitem"><p>
<code class="function">hb_unicode_general_category_func_t</code>:
returns the General Category (gc) of a code point.
</p></li>
<li class="listitem"><p>
<code class="function">hb_unicode_mirroring_func_t</code>: returns
the Mirroring Glyph code point (for bi-directional
replacement) of a code point.
</p></li>
<li class="listitem"><p>
<code class="function">hb_unicode_script_func_t</code>: returns the
Script (sc) property of a code point.
</p></li>
<li class="listitem"><p>
<code class="function">hb_unicode_compose_func_t</code>: returns the
canonical composition of a sequence of two code points.
</p></li>
<li class="listitem"><p>
<code class="function">hb_unicode_decompose_func_t</code>: returns
the canonical decomposition of a code point.
</p></li>
</ul></div>
<p>
Note, however, that future HarfBuzz releases may alter this set.
</p>
<p>
Each Unicode function has a corresponding setter, with which you
can assign a callback to your replacement function. For example,
to replace
<code class="function">hb_unicode_general_category_func_t</code>, you can call
</p>
<pre class="programlisting">
hb_unicode_funcs_set_general_category_func (*ufuncs, func, *user_data, destroy)
</pre>
<p>
Virtualizing this set of Unicode functions is primarily intended
to improve portability. There is no need for every client
program to make the effort to replace the default options, so if
you are unsure, do not feel any pressure to customize
<code class="literal">unicode_funcs</code>.
</p>
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